US3693272A

US3693272A - A floating tower for underwater dredging

Info

Publication number: US3693272A
Application number: US32993A
Authority: US
Inventors: Paul Gariel
Original assignee: Individual
Current assignee: Individual
Priority date: 1969-04-30
Filing date: 1970-04-29
Publication date: 1972-09-26
Anticipated expiration: 1989-09-26
Also published as: BE748596A; FR2043460A6; NL7006275A; GB1289289A

Abstract

A suction-dredging installation for operation at great depths and having considerably increased stability in rough water, comprising a suction nozzle in contact with the sea-bed and drawing in a mixture of water and materials, a pump installed in a floating tower of elongated form and a suction conduit coupling said suction nozzle to said pump, the tower being maintained in a vertical working position in the water by ballasting means, said pump being mounted near the lower extremity of the tower so as to obtain a great apparent depth of immersion, the pump being coupled to a delivery conduit which applies a back-pressure corresponding substantially to said depth of immersion so that the pump is capable of working without cavitation.

Description

ilirite States aten Gariel Sept. 26, 1972 [54] FLOATING TOWER FOR 3,429,062 2/1969 Nelson ..37/57 UNDERWATER DREDGING 2,347,213 4/1944 Neidl ..417/ 123 [72] Inventor: Paul Gariel, 3 we Lesdiguieres 3,544,168 12/1970 JunovIch et al ..302/ X Grenoble, France FOREIGN PATENTS 0R APPLICATIONS 1221 Filed= P 29, 1970 928,724 6/1963 Great Britain ..302/14 [21] Appl. No.: 32,993

Primary ExaminerRobert E. Pulfrey Assistant Examiner-Clifford D. Crowder [3O] Forelgn Apphcatmn Pnomy Data Attorney-Sylvester J. Liddy, John J. Hart, Joe E.

April 30, 1969 France ..6912354 Daniels and Charles BaXley May 13, 1969 France ..6914340 [57] ABSTRACT [52] 11.8. C1. ..37/63, 37/72, 302/14, A Suctiomdredging installation for operation at great 417/103 417/122 depths and having considerably increased stability in at. Cl. rough water comprising a Suction nozzle in Contact Fleld Of Seasch the Sea bed and d i g i a mixture of water 3 22 1 and materials, a pump installed in a floating tower of 17/1 elongated form and a suction conduit coupling said suction nozzle to said pump, the tower being main- [56] References C'ted tained in a vertical working position in the water by UNITED STATES PATENTS ballasting means, said pump being mounted near the lower extremity of the tower so as to obtain a great 3,522,670 8/1970 FlIpse et a1. ..37/58 apparent d h f i er ion, the pump being coupled 3,438,142 4/1969 Krutem ..37/58 X to a delivery conduit which applies a back pressure 3,212,822 10/1965 Payne et a1. ..302/14 corresponding substantially to i depth of immersion 813,935 2/1906 Avery "37/58 UX so that the pump is capable of working without cavita- 3,350,798 11/1967 Nelson ..37/ tion 3,415,068 12/1968 Casey, Jr. et a1 ..37/56 X 3,456,371 7/1969 Graham et al. ..37/ 3 Claims, 3 Drawing Figures PATENTEDsEP2s I972 SHEH 1 0F 2 INVENTOR.

PAUL GAR/EL r4 Tim/av: r

FIG. 2

IN VENTOR. PAUL GAR/EL ATTORNE r BYV U FLOATING TOWER FOR'UNDERWATER DREDGING THE INVENTION This invention relates to dredges, and is more particularly concerned with a sea-going type of dredge which however, can be used to advantage in relatively shallow bodies of water.

The primary purpose of the invention is to provide a dredge of such improved stability that it is enabled to carry out dredging operations in seas of a roughness that cannot be withstood by the conventional dredge.

Known types of dredges,either of the pump type, or the bucket type, are relatively well adapted for dredging at depths of 10, 20 or even 30 meters, provided that the surface of the sea in which they are operating is fairly smooth. In exceptional cases, certain large dredges or lighters can reach depths of 45 meters and more. However, these depths appear to be limiting values which it is difficult to exceed for many reasons, if the conventional forms of these structures are adhered to. One of the reasons for suchv limitation of operation is that the behavior of the conventional seagoing dredge in rough weather leaves much to be desired and it is generally necessary to abandon dredging operations under such circumstances due to the danger involved, or to the low efficiency of workmg.

Another reason is that if it is desired to convert a standard dredger to a deep dredger, the fact that the distance between the ship and the sea-bed will be necessarily greater requires that the pipes or other members which couple the ship to the suction nozzle placed on the sea bottom be made of substantial length. Because of their added weight and cumbersomeness the danger of destruction of these added accessories, or even of the ship, due to rough water and sea currents that may be encountered, is ever present.

Still another reason is that the distance between the pump which in conventional dredges is usually mounted in the hall in the vicinity of the water level, and the bottom to be dredged, is great in comparison to the height of water representing atmospheric pressure. This results in a suction and delivery capacity which are limited by the cavitation of the pump as soon as the depth of suction exceeds the usual limits.

A dredge constructed in accordance with the instant invention is to a very great extent free from the aforesaid effects of conventionally constructed dredges because essentially it is a floating tower, suitably ballasted and containing at least part of the installations necessary for dredging. In such tower the pump is placed sufficiently low to provide the necessary backpressure to avoid cavitation, taking into account the density of the mixture pumped, the losses of pressure which are a function of the dredging depth, and the loss of pressure inherent in the suction nozzle, including those losses resulting from a disintegration operation if this is carried out by such means.

It has been found that the aforesaid arrangement has two important advantages:

The floating tower has a stability in rough seas which is much superior to that of a conventional ship, and this increased stability makes it possible to carry out dredging operations in a rougher sea than a conventional dredger can operate in.

The pump can be placed well below the level of the water, and as low as may be desired inside the tower. The back pressure which results from such placement enables dredging to be carried out at a greater depth without risk of cavitation in the pump, permits higher concentration of mixtures, or mixtures containing denser products to be employed, and enables both the diameter of the suction pipe to be reduced, and the suction device to be provided with water-injection devices for disintegration work, that are actuated by the suction of the dredging pump itself.

Considered generally, the installation of this invention has the following characteristics:

The floating tower while it may be cylindrical, frustoconical, cylindro-conical, cigar-shaped or have any other desired configuration, is always elongated in the direction of its height. Thus, for example the proportion of the total height to the diameter, for a cylindrically shaped tower is equal to or greater than 3. The tower may comprise an enlarged portion at its lower end in order to increase its stability in rough water when the swell is high or has a period close to the natural period of oscillation of the tower without such enlarged proportion. The said tower extends normally from the surface of the water to a height that is in proportion with the amplitude of the swell and of the variations in load it is to carry, especially of dredged product when this is stored in the tower. Preferably the tower is suitably compartmented to contain the dredging pump in a position as low as desired, and everything that is deemed necessary for the operation of the installation, for the life of the crew and for safety.

The tower may be brought to the location of the dredging operation in a horizontal position by the proper use of ballast tanks and can be navigated in the same way as a ship. The ability to so handle the tower is of practical advantage at the time of its construction, for the purpose of launching and during changes in the place of dredging.

The suction device or nozzle used in dredging the product is preferably coupled to the tower by a flexible, articulated or composite pipe, that extends from the top of the suction device and terminates at the inlet of the dredging pump. The length and diameter of the suction pipe is in relation to the height of the tower, the position of the pump, the maximum depth of dredging, the nature and the rate of flow of the dredged product and the delivery pressure of the pump. The suction device is held suspended by a cable which depends from a lifting device fixed on the tower and which carries both the weight of the suction device and part of the weight of the pipe. The delivery piping for the dredged product starts from the outlet of the pump and extends up to the point of utilization or discharge to waste of the product dredged.

The anchorage system for the tower is constituted by cables which are attached either to fixed points on the shore, or to anchors, or to submerged mooring weights, and comprises the winches necessary for enabling the tower to be moved over the dredging area.

The features of a dredge constructed in accordance with the invention provide certain particular advantages when dredging at the usual depths, and these advantages are enhanced for dredging at great depths.

Thus, it has been found that in conducting dredging operations at about thirty or forty meters, the stability of the tower makes it possible to carry out such operations in a rougher sea than that which can be faced by conventionally designed dredgers. The dredge of this invention therefore, has a higher coefficient of use and consequently an increased production. These advantageous results are believed to be due primarily to the stability of the tower itself, and to the flexible connection between the tower and the suction nozzle, which enables the structure to accommodate itself readily to variations in position of the bottom of the tower due to the vertical and lateral oscillations which exist in a rough sea, although they are reduced by the towers stability. By placing the pump towards the bottom of the tower at a substantial distance below the level of the water, it is possible to utilize the back-pressure to draw in a mixture which has a density higher than that capable of being dredged by usual dredging equipment. Also a mixture of the proper density to be handled can be obtained by mounting in the suction nozzle water-injection disintegration devices actuated by the dredging pump itself.

In carrying out operations at greater depths, the tower is made longer, as is also the suction system. By mounting the pump as low as possible in the tower, dredging operations can be carried out in very deep water. For example, when a mixture having a density of 1.20 is to be sucked into the vertical intake pipe, for every ten meters that the pump is placed lower in the tower, it is possible to gain a suction height of about forty meters. Thus, if we consider a tower of 70 meters in height and emergent by meters, and place the pump at 10 meters above the bottom of the tower, that is to say, at 50 meters below the water level, it will be possible to draw-in a mixture having a density of 1.20 from a depth of 200 meters. If there is used a pressure of 10 meters in operating disintegration injection water at the suction device, the pressure at the level of the pump suction will still be 10 meters of water absolute, that is to say, equal to atmospheric pressure. If the mixture to be dredged has a density of 1.10, adequate suction for the removal of such mixture can be effected in water up to 400 meters in depth.

With this arrangement, therefore, it is possible to exploit a layer of material located under the tower of more than 300 meters in thickness. By constructing a tower which is capable of carrying a pump at 200 meters below the water level, it is possible to suck up material at from 1,300 to 1,500 meters in depth. It is true that if the suction pipe departs considerably from the vertical, and if the materials dredged are not very fine, these depths must be reduced in substantial pro-.

portions, but this does not in any way modify the fundamental advantage of the system.

The aforesaid discussion is more particularly concerned with a dredging installation embodying a direct lift; that is to say an installation in which the dredged solid production passes through the pump, which must necessarily be a materials pump. Such an installation may have some disadvantages, especially when dredging at great depths. It is to be understood however, that the novel features of this invention can be used to advantage in a dredging system provided with indirect lift, that is to say with a lockchamber system in which the greater part of the solid product dredged does not pass through the suction pump, which therefore need not be a materials pump. It has been found that when the invention is embodied in an indirect lift dredging system there results a better power efficiency and a greater suction capacity. For high delivery lengths, a multi-stage pump can be employed with a single motor.

In such an indirect lift dredging system, there is preferably mounted as low as possible in the tower a first pump for clear or lightly charged water and which takes its suction from one or a number of vertical tanks that may be located inside the tower in certain arrangements and may be wound in a helix round the tower in other arrangements. The top or one extremity of the said tank or such arrangements have connected thereto a pipe forming an extension of the long dredging pipe which terminates at the suction nozzle. Such first pump delivers to the exterior, practically at the same level at which it is mounted in the tower, the water freed from the solid products deposited in such tank and gradually filling it. A valve is provided on the pipe in advance of the tank and another valve is provided on the aforesaid extension between the tank and the pump.

A second water pump, which may be mounted in the tower in the vicinity of the first pump, draws in water from the exterior body of water at the level of such second pump and injects it under pressure into the tank that has been filled with the solid product. The filled tank has been previously isolated from the dredging circuit in order to enable the extraction from it of the solid which it contains. Such extraction may be accomplished by means of a vertical conduit, which extends from the bottom end of the isolated tanks chamber, in which there is provided an appropriate device for introducing the solid material into such conduit, and then rises exteriorly of such tank up to the top of the tower. The circuit of this second water pump comprises two isolating valves which enable it to be opened during its drainage period and entirely closed during the filling period of the tank. The said tank is designed to withstand an internal depression equal to the suction of the first pump during its filling, and to withstand the internal over-pressure applied thereto by the second pump during the drainage period.

In order to ensure a continuity of operation that is as regular as possible, without having to build tanks of excessively large size, which stand-up badly under high depressions, there are preferably provided a plurality of tanks. For example, there may be formed an arrangement of six tanks, of which five are continuously on slow filling while the sixth is on a rapid emptying rate. This arrangement is especially advantageous when the operation of valves is accomplished from the top of the tower, and for housing the six tanks and the two pumping sets since it is possible to mount all the units one above the other. The use of six tanks also readily lend themselves to a construction in which the tanks are wound in a spiral round the tower, an arrangement which provides certain advantages. When the dredging flow rate is so divided between five tanks in the course of filling, the solids are enabled to properly deposit since the flow rate in each of such tanks will be lower than the limiting deposit speed. During this time, the lift flow can readily take up the deposit in the sixth tank, since the water speed there is very much higher than the limiting deposit speed.

It will be understood that in the above discussed systems, the pumps, piping, tanks, cocks and valves thereof are utilized with gate-valves or like devices (boosters, diaphragm tanks), and with regulation means such that the equilibrium of pressures may be ensured before operating the main valves in order to avoid any shocks and water-hammer liable to affect adversely the operation and behavior of the equipment. Furthermore, the. flow of the solid material into such vertical tanks will be into the upper parts thereof and preferably tangentially. Preferably also, the tanks will comprise internal devices capable of improving the decantation of fine products that are in suspension in the water, in order to reduce the quantity of such suspended residual solids passing through the extraction pump. The extraction pump should be capable of handling a certain or given quantity. The tanks which are wound in a helix will be constructed in a similar manner.

In order that the invention may be more clearly understood, non-limitative methods of construction, given by way of example and more particularly suitable for dredging thick layers of loose materials, will now be described with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic view in vertical section of a dredging installation embodying the invention;

FIG. 2 is a diagrammatic view in vertical cross section of a modified form of construction;

FIG. 3 is a partial diagrammatic, vertical sectional view of another embodiment of the invention.

The installation shown in FIG. 1 of the drawings comprises a tower 1 inside which is mounted a pumping set 2, composed of a pump of conventional construction with one or a plurality of stages. Above the pumping set 2, the tower is divided internally into a plurality of annular compartments for reasons of buoyancy, operation and safety. The lower

annular compartments

3 and 4 may be constructed as ballast chambers to receive water or a dense ballast. The annular compartments 5 may be used as ballast chambers into which air and water may be selectively pumped to manipulate the submersions of the tower in a known manner. On the suction side of the pump is fixed a flexible pipe 6 which connects the pump to the upper portion of a suction device generally designated 7. This device is suspended by a cable 8 which passes over a pulley 9 mounted on a trolley 23 which moves along a derrick l0 fixed on the top of the tower. The cable is wound on a winch 11 mounted in the upper cylindrically shaped chamber 24 of the tower.

The flexible pipe 6 is made in a single length without coupling, and is of such length that it can be wound at 12 around the base of the tower. The pipe 6 is constructed in a known manner to make it capable of withstanding the compression forces impressed thereon by the external pressure.

The suction device 7 is constituted of a cylindroconical body 25 having a hemispherical cover 26 of large size. Within the body of the suction device are a plurality of internal compartments 13 which can be filled with water or compressed air to give such device a negative or positive buoyancy according to circumstances. The suction device is traversed by a vertical pipe 14 which is connected to an extends the flexible pipe 6 down to the suction nozzle 15 provided at the bottom tip or apex of the suction device body 25. The dimensions, height and diameter of the suction device are large compared with the diameter of the central pipe 14. The height of this device is preferably related to the maximum thickness of the sea bottom layer 27 to be dredged, in order to prevent the suction device 7 from being constantly displaced and covered by falls of the material in such layer, and to enable it to be more readily freed when such falls occur. These large dimensions of the suction device enable it to lift the flexible pipe 6 and the cable 8 by simply blowing out its ballast tanks. The length of the cable 8 is not excessive, and should be such that the suction device does not apply any tensile stress on the flexible pipe 6, but solely on the cable 8. The compressed air necessary for ballasting the suction device is sent or extracted through a flexible tube 28 which runs along the pipe 6 and is connected to compressed-air station 29 of usual construction located at a suitable place in one of the compartments inside the tower, such as the area in which the pump 2 is housed.

The tower may be held in proper position for a dredging operation by four anchorage blocks 16 to which are fixed cables 17 that are suitably connected at their upper ends to anchorage winches 31 located at places at the sides of the tower preferably positioned below the surface of the water. These cables are arranged in such manner with respect to the tower that they do not interfere with the lateral movements of the suspension cable 8 supporting the suction device 7. The dredged product is delivered by the pump 2 through the conduit 18 which extends up through the central shaft formed by the

compartments

3, 4 and 5 and which is suitably elbowed to take care of the configuration it is provided with throughout its length. The upper end of the conduit 18 is suspended from gantry l9, and discharges the product into a lighter 20 located quite close to the tower.

The above described installation depicted in FIG. 1 of the drawings operates as follows:

The product or mixture being dredged from the layer 27, if it is a non-consolidated sand is usually in a condition of unbalance as it is being dredged and falls down along a bank 21. The sand at the bottom of the bank is sucked in by the suction nozzle 15, and rises through the

pipes

14 and 6 towards the pump 2 which delivers it to the lighter through the conduit 18. When the flow of solid material diminishes due to the tendency of the slope 21 to pass into equilibrium, the operator moves the trolley 23 carrying the pulley 9 towards the extremity of the derrick 10 so as to move the suction nozzle 15 in the direction of the slope 21. When the trolley reaches the end of its outward travel, and after the suction device 7 has removed the corresponding material in the layer 27 with the trolley so located, the latter is brought back against the tower. The tower is then moved forward towards the working face of the slope 21 by the use of the anchorage winches 31. The derrick 10 may be installed on the tower so that it can be turned about the axis of the tower to enable the suction device 7 to operate in a larger space without having to move the tower. The operator has available two means for regulating the absolute level of the suction device, namely, the variation in length of the cable 8 and the variation in the depth of immersion of the tower. If the water is rough, the operator can give slack to the cable 8, and'adjust the immersion of the suction device by operating the compressed air station 29 to vary its positive or negative buoyancy. By proper manipulation of these factors the cable 8 and suction device 7 can be employed as a safety device.

In the construction shown in FIG. 1, the energy necessary for the operation of the lift and force pump is produced on board the tower by the electric generating set 22 which is supplied from a fuel reserve. Another set 32 is employed to provide the energy that is necessary to operate the other services and for the crew, and especially for operating the compressed-air station 29 that is utilized in ballasting the tower and the suction device. In other installations, the energy may be brought in from the outside over an electric line. Such an arrangement is appropriate, for example, when a dredger is employed in cleaning out an artificial lake for hydro-electric energy production. The suction nozzle 15 may of course be additionally provided with conventional arrangements to provide better suction of the product, such as by nozzles 33 for injecting water under pressure in its vicinity. The water to so injected may be sent from the tower through an additional flexible conduit, or it may be brought up to proper speed for purposes of injection, in situ, by any suitable means, such as, for example, a pumping set 34 of suitable known construction mounted in the suction device itself. By acting on the

ballast chambers

3, 4 and 5, it is of course possible to vary the degree of immersion of the tower and to bring it back to the vertical if it has become tilted by the action of the cables. Care will obviously be taken not to raise the center of gravity too close to the metacentric height in order to preserve a good margin of safety in the equilibrium of the tower.

The construction shown in FIG. 2 of the drawings is better adapted to dredging operations conducted at great depths. In this construction the flexible suction pipe 6 connected to the suction device 7' is constituted of a plurality of elements or sections which are stored along the inside wall of the tower in the manner illustrated by the sections 6". The stored section6" are handled by means of a cable 35 connected to a suitable winch 36 mounted in the upper compartment 24' of the tower I. The first of such sections is connected to the vertical pipe 14' of the suction device 7'. The coupling of succeeding pipe sections is then effected in the compartment 24 above a well 37 formed centrally of the annular compartments 3, 4', 5' and constituting a fluid-tight passage, which is located above the water level. After a pipe section 6" has been connected it is lowered down inside the well 37. The last element or section 6" to be connected has its upper end coupled in a fluid-tight manner to the base of the tower at 38. The upper end of the last pipe section therefore is in communication with the bottom 39 of the well 37. The material pump 2' is also connected to such well bottom 39. The pump therefore, will draw in the mixture obtained from the sea-bed through the pipe 14' of the suction device 7 and the sections of the pipe 6 and into the well bottom 39 from whence such material will pass through the pump 2' and the conduit 18' to be discharged into the lighter 20. For reasons of flow stability, there is an advantage in closing the bottom of the well 37 by a full bottom partition or wall 40 during the dredging operation, so as to form such well bottom 39 into a closed chamber, as illustrated.

It will be noted that in the embodiments shown in FIGS. 1 and 2, the dredger is assumed to be constructed vertically at a place close to the place of use, so that it can be transported easily to the latter for the dredging operation. The dredger of this invention, however, may be built in a dry dock at some distance from its place of working and brought horizontally to such place. Precautions are taken during the design and construction of the tower to make sure that it does not sink as a result of the filling of the central well during such transportation or for accidental reasons, such as fracture of a conduit, bursting of a pump, etc. When the tower arrives at the place where the dredging operation is to be conducted it may readily be tilted so as to bring it vertically into the proper working position by utilization of the ballast tanks, it being understood that the movements of the ballast into such tanks are carried out in a given sequence.

While the use of the aforesaid embodiments have been described and illustrated in connection with the dredging of loose sand, it will be understood that they may of course be arranged for dredging all products, whether consolidated or not. If the product layer to be dredged is in the least solid, it may be necessary as has been indicated to additionally provide the suction device with means for injecting water under sufficient pressure to break up the product before sucking it up into the pipe and through the pump to the lighter. If such type of means is incapable of effecting the desired result, the suction device may additionally be provided with a mechanical disintegrator driven by a motor incorporated in the suction device in the manner of the motor 34 in FIG. I. The power required for the operation or such motor may be supplied from the tower through an appropriate liquid conduit or electric cable.

FIG. 3 is a diagrammatic view in vertical section of the lower part only of a dredging tower in which is contained a modified form of suction and delivery system for the dredged product. For the sake of simplification there is not shown in the bottom or lower end 41 of the tower of FIG. 3 any of the compartments referred to in connection with FIG. 1, but which may of course be provided also in this embodiment.

The system shown in FIG. 3 for lifting the dredged products from the ocean layer to the lighter, is essentially constituted of a main lifting pump 42 with which are associated two

tanks

43 and 44, which are practically identical, a secondary lifting pump 45 and the necessary piping systems, cocks and valves. It should be noted that the pumps, pipes and tanks in this system are always full of water.

The operation of the aforesaid parts takes place in two stages. In the first stage the tank 43 becomes filled with solid products, while the solid products contained in the tank 44 are being evacuated. In the second stage, this process is reversed, the tank 44 fills with solids while the tank 43 is emptied of solids. The operation may be detailed as follows:

During the first stage, which is the condition shown in FIG. 3, valve 46 mounted on the upper end of the tank 43 is open and a valve 47 mounted on the upper end of the tank 44, is closed. A valve 48 provided on a branch connecting the upper end of the main dredging conduit 50 to tank 43, is open and a valve 49 provided on a second branch connecting the upper end of conduit 50 to the tank 44, is closed. A valve 51 provided on a pipe 52 connecting the upper end of the tank 43 to the pump 45 is closed, and so also is a valve 53 on a pipe 54 extending through the bottom of tank 43 and into the solids deposited in such tank and connected at its upper end to the main vertical discharge pipe 55. A valve 56 on the lower section 57 of the main discharge pipe 55 below the juncture of the pipe 54 there with is open, and so also is a valve 58 on piping 59 connecting the upper end of tank 44 to the pump 45. The lower end of the pipe section 57 extends through the bottom of tank 44 and into the solids deposited in such tank. The bottom of each of the

tanks

43 and 44 is provided with a conventional device 60 for facilitating the introduction into the conduits or

pipes

54,57 the solids contained in such tanks during the extraction of such material from such tanks. The main pump 42 during this first stage creates in the tank 43 a depression or vacuum condition sufficient to cause the upward movement in the main dredging conduit 50 of the mixture composed generally of sea water and solids such as, pebbles, ores and coarse sand. This mixture passes through the open valve 48 and enters the top portion of the tank 43. As indicated, such entry is preferably caused to be made in a tangential manner in order to facilitate the separation of the solid products from the water which accompanies them. The solids fall and deposit in the tank 43 in which their level progressively rises. At the same time, the water, freed from the greater part of the solids, rises in a chimney 61 mounted in the upper end of the tank 43 and passes through the valve 46, and into the main pump 42. The pump imparts to the water the necessary pressure to discharge it directly into the sea through a delivery pipe 62. A non-return valve 63 is advantageously mounted on the outlet end of pipe 62 in order to prevent any undesirable entry of sea water into the tower in the event of the fracture of any of the piping systems. By virtue of the fact that the water has been well decanted in the tank 43, the main pump 42 does not need to be a materials pump, but may be a simple water pump. However, it is well to assume that there will always remain fine particles in suspension in the water. Accordingly, the pump should have a certain resistance to abrasion.

While the tank 43 is filling with solids, the tank 44 which has been filled in a previous cycle of operation of the system, is being emptied of the solids which it contains. During this time, the

valves

47, 49, 51 and 53 are closed and the

valves

56 and 58 are open, as previously stated. With the valves so arranged, the secondary lifting pump 45 takes water from the sea at its level through its suction pipe 64 and delivers it through the valve 58 and pipe 59 into the upper portion of the tank 44. The entry of such water into tank 44 forces the solid material that has settled therein into the lower end of the conduit 57. The solids are carried away in this conduit and pass through the valve 56 and into the vertical main pipe 55 which conveys them up to the top of the tower 41, from where they are sent to storage which may be the lighter 20 of FIG. 1.

During the second stage or next cycle of operation of the system, all the aforesaid valves are operated to positions which are the reverse of those to which they were set for carrying out the first stage. There naturally exists certain difficulties in passing from one stage to the next uniformly and rapidly, but without shocks, without water-hammer, without wear of the valves and without blocking up the piping systems. In order to avoid such happenings, the installation is provided with a control board 65 designed to cause each member to act at the right time, so that its operation may take place under the desired conditions of time, sequence and rapidity. This control panel 65 comprises outlets corresponding to the number of valves. In order to avoid complicating the drawings, the connections to the valves are not shown, but each of the outlets are provided with the number of the corresponding valve to which it is connected. One method of passing from one stage to the other consists in first closing all the valves concerned with the

secondary lifting circuit

51, 53, 56 and 58, the conduit 55 no longer containing any solids. Then the valve 49 is opened, and the valve 47 and, after a certain interval, valve 46 is closed and then valve 48. These operations can be facilitated in certain cases by the use of circuits (not shown in the drawing) for balancing the pressures, since while the tank 43 is for example under depression with respect to the surrounding medium, the tank 44 is under a pressure higher than this same medium. A previous balancing operation facilitates the actuation of the valves.

The arrangement shown in FIG. 3 is the most simple one of the two stage type. When thesystem is to be used for larger dredging installations, one, two or even three installations such as that above described may be installed in the tower, but preferably with a single main lifting conduit 50. These modified arrangements have the advantage of safety of utilization. They have the disadvantage however, of increasing the number of pumping sets at work. For dredging installations which have very large flow rates of solids at great depth, there may be an advantage in utilizing a single multicellular main pump such as 42, and a single secondary pump such as 45, and substituting for the two

tanks

43 and 44, a greater number of tanks, such as for example six. in such an arrangement five tanks may be placed on slow filling while the sixth tank is placed on rapid drainage. This system offers the advantage of reducing the unitary dimensions of the lock chambers and the valves. In addition, it reduces the risk of stoppage of the flow in the main lifting conduit 50 during the change from one stage to the next, since by stopping the flow into one tank, one-fifth of the total flow is removed from the corresponding piping system. For a short time, each of the other four tanks on filling has its flow-rate increased by only 25 percent with respect to its nominal flow-rate, which will disturb the lift very little and reduce wear.

In the embodiment depicted in MG. 3, the

tanks

43 and 44 are shown inside the dredging tower. As has previously been indicated, in other arrangements according to necessary invention, tanks such as 43 and 44 may be placed outside the tower and therefore in the surrounding water. Such an arrangement has certain advantages,- for example, in the maintenance of the installation, or in order to reduce the diameter of the tower at its lowest part, thereby rendering the solution of the problems of its resistance to compression stress less difficult. Also, instead of placing the main pump 42 above the

tanks

43 and 44, it may be placed to equal advantage at the level of such tanks, or even below them. In certain cases this may even be necessary in order to avoid cavitation in the pump. The construction of the arrangement of FIG. 3 can be made still simpler by using the bottom of the tower as a single tank. In such a construction, the operation of the dredging installation will then be intermittent, since the lift of materials will have to be stopped during the drainage stage of the tank.

What is claimed is:

1. Apparatus for underwater dredging in a body of water comprising a floating device, and means for collecting solid material from the bed of the body of water including separator means supported by said floating device below the water level for separating a mixture of solid material and water and having a top portion for containing the separated water and an underlying portion for containing the separated solid material, inlet conduit means connected to said top portion of said separator means to discharge thereinto solid material raised upwardly with water from the bed, second means connected to the top portion of said separator means and in communication with the discharge end of said inlet conduit means at the top portion of said separator means and operative simultaneously to create a suction in said top portion sufficient to raise solid material and water from the bed through said inlet conduit means and into said separator means, to decant through the top of said separator means water which has been separated out from the solid material and is contained in the upper portion of said separator means, and to discharge such decanted water back into the body of water, normally closed discharge conduit means connected to a receiver for solid material and to the bottom end of said underlying portion of said separator means for removing therefrom the separated solid material contained in such underlying portion, said top portion of said separator means being defined by a cylindrical interior surface to which the discharge end of said inlet conduit means is disposed so that the water and solid material raised in said inlet conduit means is discharged into said top portion substantially tangentially to such cylindrical interior surface thereof so as to facilitate the separation of the solid material from the water, said second means including an outlet chimney member which extends through the top of said top portion in substantially coaxial relation to said cylindrical interior surface, and in which the separated water rises as the solid material settles in said underlying portion of said separator means, normally inoperative means connected to the top portion of said separator means for taking water from the body of water and injecting it into said top portion under such pressure as to drive the separated solid material contained in the underlying portion of said separator means into and through said discharge conduit means to the receiver, and control means operative to stop such simultaneous operations of said second means and to close said inlet conduit means when said underlying portion of said separator means has been sufficiently filled with solid material, and to open said discharge conduit means and to start the operation of said injecting means to evacuate the solid material from the separator means.

2. Apparatus as defined in claim 1, in which said floating device is substantially cylindrically shaped and has a substantially cylindrically shaped upper portion projecting above the surface of the body of water, in which said inlet conduit means depends substantially axially from the bottom end of said device, and including supporting means revolvably mounted on said projecting upper portion for movement about the vertical axis of said floating device, and means depending from said supporting means for supporting on the latter a submerged portion of said inlet conduit means.

3. Apparatus as defined in claim 1, in which said separator means includes a plurality of vertically disposed separator units each having a top portion for containing separated water and an underlying portion for containing separated solid material, said inlet conduit means, said second means, and said injecting means being common to and connected to the top portions of both of said units, and said discharge conduit means being common to and connected to the underlying portions of both of said units, said control means being operable to cause said inlet conduit means and said second means simultaneously to discharge raised solid material and water into the top portion of one of said units, to decant separated water from the top portion of said one unit and to discharge back into the body of water, separated water which has decanted from said one unit, and to stop the discharge of raised solid material and water into the other of said units, and said control means being operable to cause said discharge conduit means and said injecting means to evacuate by the pressure of water drawn from the body of water the solid material drawn from the bed and deposited in said other unit prior to the stoppage of the discharge of solid material and water raised from the bed into said other unit, while said one unit is being filled with solid material raised from the bed.